Threat modeling: why would a https remote endpoint be prone to privilege escalation?

Question

I'm currently creating a threat model for a web application and have a http proxy at the edge of the trusted network. It's communicating with Google Maps via the API, using a https connection; so this connection is crossing a trust border. I'm using Microsoft's free SDL threat modeling tool in the 2016 version. After modeling the entities I checked what possible attack surfaces it generates.

One of those threats I do not understand. It says "An attacker may pass data into HTTP proxy in order to change to flow of program execution". What is this refering to? This proxy only accepts http(s) requests and forwards them for a client and does the same with the answers from outside (gateway setup). The only possible explanation I could think of would be a vulnerability in the proxy software/OS, e.g. an overflow exploited by manipulated http(d) packets or headers. Are there any possible other explanations I am missing?

Of course this threat generation is generic and treats my http proxy only as a "generic process" so it might not be applicable at all and is only referring to a threat existing if two processes are communicating on the same machine (or by RPC) and not over a network interface.

Thanks and regards

Answer 1

If someone compromises the proxy, they can attack your web app.

If they're on the same machine (inside all the same boundaries) then this matters less, not more. But the proxy is probably more exposed to attack.